Site selective growth of nanowire lasers, in particular on silicon, has a large potential and may become an important building block of future photonic devices. In particular, semiconductor III-V nanowires have the potential to create a new generation of lasers and on-chip coherent light sources by virtue of their ability to operate as single mode optical waveguides, resonantly recirculate optical fields and provide gain. Furthermore, the small nanowire footprint facilitates direct growth on silicon, as has been demonstrated by T. Martensson et al., “Epitaxial III-V Nanowires on Silicon”, Nano. Lett. 4 (2004) 1987, and J. Treu et al., “Enhanced Luminescence Properties of InAs—InAsP Core-Shell Nanowires”, Nano. Lett. 13 (2013) 6070. However, the low refractive index contrast between the nanowire and the silicon substrate provides poor modal reflectivity and hampers the lasing operation. For instance, for GaAs—AlGaAs nanowires grown on silicon, the modal reflectivity at the GaAs-silicon interface is typically below 1%, and only by removing the nanowires from the growth substrate reflectivities can become large enough to achieve lasing, as demonstrated by B. Mayer et al., “Lasing from Individual GaAs—AlGaAs Core-Shell Nanowires up to Room Temperature”, Nature Photonics 4, 2013. However, removing the nanowires from the growth substrate and transferring them to a different substrate is a complex and error-prone nanofabrication process, and has hindered the progress towards the mass-production of integrated nanowire-based photonic devices.
Recently, tapered InGaAs nanopillars supporting higher order helical optical modes have been shown to lase on silicon, cf. R. Chen et al., “Nanolasers Grown on Silicon”, Nature Photonics 5 (2011) 170, and H. Sun et al., “Nanopillar Lasers Directly Grown on Silicon with Heterostructure Surface Passivation”, ACS Nano. 8, 2014. However, their comparatively large footprint and multimode resonator structure typically lead to fairly low spontaneous emission coupling factors in the order of β=0.01, and, consequently, high pump thresholds for lasing and a complex far-field radiation pattern.
What is needed is an integrated nanowire laser structure that has a high coupling factor and is easy and efficient to manufacture.